The present invention relates to an electron emitting element and a switching circuit using the electron emitting element. More particularly, the invention relates to an electron emitting device for use in effecting a switching operation in a linear region, and also to a switching circuit using the electron emitting device.
In recent years, electron emitting elements of field-emission type have been developed, by applying the advanced Si semiconductor microfabrication technique. These elements are as small as semiconductor devices and are expected to be used in flat panel displays and the like. A representative type of an electron emitting element is disclosed in C. A. Spindt et al., Journal of Applied Physics, vol. 47, 5248 (1976).
In the conventional electron emitting element, a positive voltage is applied to the gate electrode, thereby generating an intense electric field at the emitter. The electric field attracts electrons from the emitter into a vacuum. Meanwhile, a positive voltage higher than the voltage applied to the gate electrode is applied to the anode electrode, which opposes the emitter. The electrons moving from the emitter toward the gate electrode are thereby attracted to the anode electrode. Thus, the electrons emitted from the emitter are collected at the anode electrode.
In the electron emitting element, the number of electrons emitted from the emitter is determined by only the intensity of the electric field generated at the tip of the emitter. Generally, the anode electrode is much more spaced from the emitter than the gate electrode is spaced therefrom, so that the number of emitted electrons, i.e., current is determined only by the gate voltage. If the anode voltage is low, the electrons emitted from the emitter are strongly attracted to the gate electrode. As a result, the electrons move to the gate electrode and do not reach the anode electrode.
As the anode voltage increases, the number of electrons moving toward the gate electrode decreases and the number of electrons moving toward the anode electrode increases. When the anode voltage becomes sufficiently high, all electrons emitted from the emitter reach the anode electrode.
The operating characteristics of the electron emitting element, described above, make no problem when the element is used in an apparatus, such as a flat panel display, wherein the anode current is controlled by the gate current. The characteristics make a problem, however, when the element is used as a power switching element by utilizing the high insulating property of a vacuum.
When the electron emitting element is used in a switching circuit, it is connected between a load having an impedance Z and a power supply having a voltage V.sub.0. If the element performs switching operation in its saturation region, the anode voltage will increase, inevitably causing a large power loss.
Moreover, the electron emitting element lacks in operating reliability due to the difference between its characteristics and its design characteristics. This is particularly because the anode current is determined by the gate voltage, irrespective of the size of the load and the power-supply voltage.
Due to its above-mentioned disadvantageous aspects, the electron emitting element is operated in its linear region when it is used in a switching circuit to emit electrons. When the electron emitting element is operated in its linear region, the gate current is of the same order as the anode current while the element remains on during the switching operation. Consequently, a large power loss takes place at the gate, which should be controlled. Further, the element is likely to break down since an excessive gate current flows.
An electron emitting element for use in a switching circuit has an array of conical emitters. It is difficult to sharpen the tips of the emitters in the same manner and to space them from the gate at the same distance. It is therefore impossible for the conical emitters to emit electrons in the same way. More specifically, the emitters start emitting electrons at different gate voltages. Thus, as the gate voltage is increased, the currents flowing in some emitters reach an upper limit before the currents flowing in the other emitters reach an upper limit. As a consequence, the first-mentioned emitters are short-circuited to the gate. Current inevitably flows between the emitters and the gate, disabling the emitter array to perform its function.
An electron emitting element that is free of this problem is disclosed in Ghis et al., IVMC90 Technical Digest. This element comprises a glass substrate, a mesh-shaped emitter lines provided on the substrate, and a resistor layer covering the emitter lines. The element further comprises an SiO.sub.2 layer provided on the resistor layer, a gate layer formed on the SiO.sub.2 layer, and conical emitters arranged on the Mo layer.
In this electron emitting element, the resistor layer is interposed between the emitter lines and the emitters. Hence, as each emitter outputs a current, its potential increases, decreasing the potential difference between the emitter and the gate. As a result, the output current of the emitter decreases. Namely, the resistor layer performs negative feedback. The negative feedback works on some emitters which start emitting electrons at a low gate voltage more strongly than on other emitters, which starts emitting electrons at a higher gate voltage. Thanks to the negative feedback, it becomes difficult to short-circuit between the emitters and the gate for making all emitters have the same electron-emitting characteristic. Even if any emitter were short-circuited, the emitter array would not be disabled to operate. This is because the resistor layer receives the gate voltage.
The electron emitting element having a resistor layer provided near the emitters works well in a flat panel display or the like in which the element needs to generate a relatively small current. When the electron emitting element is used as a power switching element operating by utilizing the high insulating property of a vacuum, however, a large current flows in the resistor layer, causing a great power loss.
If the electron emitting element is incorporated in a flat panel display or the like, the anode voltage can remain much higher than the gate voltage, and most of the electrons emitted from the emitters move toward the anode electrode. If the electron emitting element is used as a switching element, however, the anode voltage falls to almost the gate voltage when the element is switched on. In this case, an excessive current flows in the gate electrode, possibly breaking down the element, since the resistor layer provided near the emitters does not serve to prevent the breakdown of the element.
If the conventional electron emitting element is used in a switching circuit and operated in its linear region, the gate current will be as large as the anode current when the element is switched on. Consequently, a large power loss will occur in the gate electrode for controlling the switching. Furthermore, the excessive gate current is likely to break down the element as a whole.
As describe above, if the electron emitting element of field-emission type is used as a switching element, a large power loss will occur in the gate electrode, and an excessive current will flow in the gate electrode, possibly breaking down the element as a whole.
If the electron emitting element, which has a resistor layer, is used as a switching element, a large power loss will occur in the resistor layer, because the resistor layer is provided near the emitters. When the element is switched on, the anode voltage falls to almost the gate voltage, whereby an excessive current flows in the gate electrode, possibly breaking down the element.